1. Field of the Invention
The invention pertains to a vibration damper of the type having a pressure tube filled with a damping medium, a displacer which is axially moveable in said pressure tube, and a damping device which produces a damping force as a function of the movement of the displacer and the flow of damping medium through the damping device.
2. Description of the Related Art
In a vibration damper, a reactive force is transmitted to the vehicle body as a function of the spring travel of the wheel. The forces which thus occur can reach a considerable level. For this reason, vibration dampers are already designed so that a much weaker damping force is produced in the inward direction of the spring travel of the wheel than in the outward direction. So that all load requirements can be met, the body parts which support the vibration damper must be much stronger than other body assemblies; accordingly, these parts are made, for example, out of sheet or plate which is much thicker than that used for other parts. This increase in the amount of material is unsatisfactory in terms of production engineering and is also associated with a weight disadvantage. If this effort were not made, however, the connection between the vibration damper and the body could break, and in particular in the case of a motor vehicle with shock-absorbing strut units, it would then be impossible to steer the vehicle. Spring travel of this magnitude, however, occurs only in very rare cases, e.g., in an accident, when the vehicle swerves against a high curb or slides through a ditch.
Designs which are intended to handle overloads are also known from the area of gas springs. U.S. Pat. No. 3,995,842 discloses a gas spring in which a predetermined breaking point is provided in the piston rod or in the cylinder. In the case of a vibration damper, however, a predetermined breaking point of this type would allow the oily damping medium to escape from the vibration damper and thus to become a burden to the environment. After the piston rod has broken, furthermore, it may under certain conditions be impossible to steer the vehicle.
DE 42 36 150 describes a gas spring in which a piston ring is deformed out of its groove when a defined load is exceeded, with the result that the two working spaces of the cylinder become connected to each other.
There are many different types of damping valves in vibration dampers which, on the basis of their design, have a degressive damping force characteristic. Thus, in a certain range, only a very moderate increase in the damping force is reached relative to the spring velocity of the wheel. The degressive damping force characteristic is no longer in effect at the spring velocities which occur when extreme obstacles are encountered; in this situation, the flow channels act instead progressively on the damping forces. See, for example, U.S. Pat. No. 5,509,512.
U.S. Pat. No. 5,441,132 discloses a double tube vibration damper, in which a compensation element is installed between the casing tube and the pressure tube to compensate for the temperature-related differences in length attributable to the different materials used for the casing tube and the pressure tube. The spring force of the compensation element acting on the piston rod guide or on the pressure tube is oriented in such a way that the piston rod guide and the casing tube are always braced against each other.